I need to make sure about the type of communication that this board camera works with.

Its user manual recommends connecting RxD and TxD of the camera board to the pins of the UART-USB board.
I assume this means that the camera board itself (without the UART-USB board) works with serial communication of a USB. am I correct?

RB-Lin-48 uses the UART (a.k.a. serial) communication protocol. This particular model used the RS-232 signal standard which means it can be directly connected to older DB9 ports, but would not work directly with TTL signals.

The depicted USB-to-serial adapter is simply used to allow a modern computer (which do not include serial ports anymore) to communicate with the device. Any device capable of serial communication (at RS-232 voltage levels in this case) will be able to communicate with the camera.